The present invention generally relates to thick film resistors, methods of trimming the thick film resistors, and printed circuit boards having the thick film resistors which are trimmed according to such methods. More particularly, the present invention relates to a thick film resistor which may be trimmed (subjected to a function trimming process) by irradiating a laser beam so as to adjust the resistance in a state where the thick film resistor is mounted on a printed circuit board, a method of trimming such a thick film resistor, and a printed circuit board having a thick film resistor which is trimmed according to such a method.
Generally, a thick film resistor is used as a miniature type resistor which is mounted on a printed circuit board. The thick film resistor is sometimes referred to as a chip resistor. The thick film resistor has a predetermined resistance when it is manufactured. However, when the thick film resistor is assembled on the printed circuit board together with other circuit parts, it is necessary to finely adjust the resistance of the thick film resistor so that the circuit as a whole has a predetermined characteristic in relation to the other circuit parts. The resistance of the thick film resistor is finely adjusted in a state where the thick film resistor is coupled to the circuit which is constituted by the other circuit parts, while measuring the resistance of the thick film resistor, or while measuring a voltage across a reference point and a predetermined point in the circuit which is located on the printed circuit board and is coupled to the thick film resistor. The former method of trimming the thick film resistor while measuring the resistance of the thick film resistor, is often referred to as an element trimming method. The latter method of trimming the thick film resistor while measuring the voltage across the reference point and the predetermined point in the circuit which is located on the printed circuit board until the measured voltage becomes equal to a predetermined voltage, is often referred to as a function trimming method. The trimming of the thick film resistor so as to finely adjust the resistance of the thick film resistor, is performed by removing a portion of a resistor body of the thick film resistor.
Conventionally, as a method of performing such a trimming process, there was a so-called sand-blast trimming method. According to the sand-blast trimming method, glass powder is blasted onto the resistor body by use of a nozzle, and a portion of the resistor body is scraped off by the friction between the glass powder and the resistor body. However, according to this sand-blast trimming method, a thin layer of the resistor body remains on a substrate of the resistor body after the trimming process is performed, and in this case, a crack is easily formed in this thin remaining layer of the resistor body. As a result, the resistance changes as the resistor body ages, and a drift is introduced in the current which passes through the resistor body. Thus, there is a problem in that it is impossible to accurately and finely adjust the resistance of the resistor body.
On the other hand, a laser trimming method was developed. According to the laser trimming method, a side surface of the resistor body which is trimmed, is approximately perpendicular to the substrate of the resistor body. Hence, compared to the sand-blast trimming method, it is possible to finely adjust the resistance of the resistor body with more accuracy. Therefore, the laser trimming method is the more popularly used method.
Conventionally, when using the sand-blast trimming method or the laser trimming method, the thick film resistor is placed on a trimming board. As will be described later on in the specification in conjunction with a drawing, the thick film resistor is electrically coupled to a printed circuit board (already having other circuit parts mounted thereon) on which the thick film resistor is to be mounted, by use of lead wires. Then, the resistor body of the thick film resistor is trimmed while measuring the voltage across a reference point and a predetermined point in a circuit which is located on the printed circuit board and is coupled to the thick film resistor.
In the conventional thick film resistor, the resistor body is formed over the full width of the substrate of the thick film resistor, and a pair of electrodes are provided on both ends of the resistor body. In addition, the energy of the laser beam is generally higher at a point when the laser beam first hits a target than the energy of the laser beam at a point thereafter. Thus, when the trimming process is performed by use of the laser trimming method in a state where the thick film resistor is mounted on the printed circuit board, for example, the resistor body of the thick film resistor is subjected to a rapid temperature change at a predetermined portion where the laser beam first impinges on the resistor body and at a portion in the vicinity of the predetermined portion. For this reason, a crack is formed in the vicinity of the predetermined portion where the laser beam first impinges on the resistor body, and it is impossible to perform an accurate and stable trimming of the resistor body.
When the thick film resistor is mounted on the printed circuit board and the position where the laser beam first impinges is selected to a position on the printed circuit board, the printed circuit board generally cannot withstand the laser beam because the printed circuit board is usually made of paper-phenolic plastic. As a result, the printed circuit board becomes damaged at the portion where the laser beam impinges. For this reason, as described before, there was a method of trimming the resistor body of the thick film resistor by placing the thick film resistor on the trimming board which is made of a material which will not be damaged by the laser beam which impinges thereon, and by selecting the position where the laser beam first impinges to a position on the trimming board. According to this method, the trimming process is performed in the state where the thick film resistor is placed on the trimming board.
The thick film resistor which is subjected to the trimming process, is disconnected of the lead wires, and is removed from the trimming board. The thick film resistor is then mounted at a predetermined position on the printed circuit board, and is permanently fixed to the printed circuit board by soldering.
However, according to the conventional thick film resistor and the conventional method of trimming the resistor body of the thick film resistor, the trimming process must be performed in the state where the thick film resistor is placed on the trimming board. Further, the thick film resistor which is subjected to the trimming process, must then be mounted on the printed circuit board. Hence, the operations of trimming and mounting the thick film resistor are troublesome to perform, and the productivity of the thick film resistor is accordingly poor. Therefore, there is a disadvantage in that it is impossible to manufacture the printed circuit board which is mounted with the thick film resistor, at a low cost.
On the other hand, there is a so-called hybrid integrated circuit (IC) or the like in which the resistor body and the electrodes of the thick film resistor are printed, together with other wirings, on a circuit board which is made of ceramics and is mounted with ICs or the like. The ceramic circuit board will not be damaged by the laser beam. Hence, the position where the laser beam first impinges may be selected to a position on the ceramic circuit board, and the resistor body is trimmed by moving the laser beam. According to this method, there is an advantage in that the thick film resistor which is subjected to the trimming process, need not be moved and mounted on the circuit board, as in the case of the conventional method described before. However, the ceramic circuit board is fragile. Moreover, the ceramic circuit board is expensive compared to the plastic circuit board, since ceramics is used for the main plate body. In addition, the ceramic circuit board must be subjected to processes such as printing, baking, and molding, and the manufacturing cost of the ceramic circuit board is high also due to the large number of manufacturing processes. Furthermore, when the trimming of the resistor body is imperfect, the whole ceramic circuit board must be destroyed as being defective, since the thick film resistor is printed on the ceramic circuit board and cannot be removed from the ceramic circuit board. Thus, in addition to the fragility of the ceramic circuit board, the yield of nondefective ceramic circuit boards is poor, and the manufacturing cost of the ceramic circuit board is high also from these points of view.